Percutaneous bone screw device and method

ABSTRACT

Embodiments of the invention provide a bone screw device including a bone screw with a temporary encasement. In some embodiments, the temporary encasement comprises a biocompatible material that may be an osteoinductive, a hemostatic and or a bacteriocide. Some embodiments include a temporary encasement including a therapeutic. In some embodiments, the temporary encasement comprises a wax. In some embodiments, the screw head is substantially U-shaped. In some embodiments, the screw includes an extension tip extending from the screw shaft. Some embodiments of the invention include a system for percutaneous implantation of a bone screw including a screwdriver shaft rotatably coupled to a screw that includes a temporary encasement. In some embodiments, the screw shaft can be at least partially rotated by the screwdriver shaft. Some further embodiments further include a retractable sleeve at least partially surrounding the screwdriver shaft and the bone screw.

BACKGROUND

Bone screws generally comprise an implantable screw capable of internalfixation or anchoring, and can be used to surgically correct many typesof injuries or deformities including scoliosis or kyphosis. Bone screwscan be used to repair a fracture, or to secure rods, plates or nails,and can be permanent, semi-permanent or bio-degradable. For example, abone screw may be left in place after a patient heals, or the bone screwmay be surgically removed.

Different types of implants (e.g., screws, pins, rods) are sometimesused in spinal surgery to help fix the spine into a more normal positionand stabilize a specific area of the spinal column. For example, acortical screw is a type of bone screw designed to screw into bonyprominences of the posterior vertebra or other bone where dense corticalbone is predominantly present, whereas a cancellous bone screw, whichhas a different thread pattern, is sometimes used during surgery tosecure the anterior vertebra or other bone where porous cancellous boneis present in greater quantity. Functioning as firm anchor points, twoor more bone screws can be interconnected using locking rods, allowing aspinal segment to be fixated for stabilization or fusion.

Screws for internal fixation can be surgically inserted either open orpercutaneously. If they are inserted open, the skin, muscle, andconnective tissue are split and retracted surgically, providing openexposure to the underlying bone. Screws and other hardware (e.g., tointerconnect the screws) are then implanted, after which the muscle andskin are re-approximated and surgically closed (using sutures or othermethod).

The most common method for percutaneous insertion of screws for internalfixation is the use of cannulated screws with surgical guide wires. Theguide wire, which has a sharp tip that may also be threaded, is mountedto the end of a drill and surgically inserted under fluoroscopicguidance or image guidance until it penetrates through skin and muscleand anchors into bone. Small diameter (1.5 mm or less) guide wires aretypically used so that they can be disengaged and repositioned withoutsubstantially damaging bone if they do not strike the bone in thedesired target. Tools such as dilators, drills, and taps that arecannulated are then positioned over the anchored guide wire and forceddown into contact with the bone. These tools are used to prepare thehole to accept the screw. A cannulated screw is then positioned over theguide wire and a cannulated screwdriver is used to insert the screw intobone while the guide wire stays in position. After screw insertion, theguide wire is removed and the puncture required to place the guide wireand screw is sealed.

Pins, screws, and rods can also be used for external fixation. In thiscase, a portion of the screws and pins reside outside of the body, butpass through the skin and muscle to connect to bone, usually at somedistance from the injury or surgical region.

Whether used for internal or external fixation, percutaneous insertionand placement of a bone screw can result in tissue injury. For example,during the insertion procedure, the tap passed over the guide wire orthe screw threads may become entangled with connective tissues betweenthe entry point and a bone fixation region which may lead to traumaand/or infection. Moreover, the percutaneous placement of screws canresult in the entanglement, entrapment, laceration or compression of oneor more nerve fibers that may lead to severe pain, nerve palsy and/orparalysis.

In some cases, using a dilator tube through which the screw is insertedcan mitigate this problem. A tube (or sequence of successively largertubes) is inserted through muscle and connective tissue to form achannel to a position on a bone. A screw can then be passed through thetube and driven into the bone without contacting soft tissues. However,this method can be tedious and it can be difficult to introduce thetube, which must have an inside diameter adequate to pass the screw andscrew head, and a substantial wall thickness for tube strength.

SUMMARY

Some embodiments of the invention include a bone screw device comprisinga screw including a screw shaft, a screw head and screw tip coupled tothe screw shaft. The screw shaft includes a screw thread, and atemporary encasement at least partially covering the screw. In someembodiments, the temporary encasement envelops substantially all of thescrew thread. Some embodiments include a temporary encasement comprisinga substantially symmetrically curved surface extending over the screwtip.

In some embodiments, the temporary encasement comprises a biocompatiblematerial. The biocompatible material can be an osteoinductive, ahemostatic, and/or a bacteriocide. Some embodiments include a temporaryencasement comprising a therapeutic, and in some embodiments, thetherapeutic is dispersed within the temporary encasement. In someembodiments, the temporary encasement comprises a wax.

In some embodiments, the screw shaft further includes a proximal end anda distal end, and the screw head is substantially U-shaped and coupledto the proximal end. In some embodiments, the screw includes anextension tip comprising a shaft including a screw thread, a tipproximal end coupled to the screw distal end, and a tip distal endextending from the distal end of the shaft.

Some embodiments of the invention include a system for percutaneousimplantation of a bone screw. In some embodiments, the system caninclude a bone screw comprising a proximal end and a distal end, a screwshaft, a screw head, and screw tip coupled to the screw shaft. The screwshaft can comprise a shaft proximal end and a distal shaft end and caninclude a screw thread, and a temporary encasement at least partiallycovering the screw. In some embodiments, the screw shaft is configuredand arranged to be at least partially rotated by a screwdriver.

In some embodiments, the screw head is substantially U-shaped andcoupled to the proximal shaft end, and the screwdriver is coupled to aninner surface of the screw head. Some embodiments include a sleeve atleast partially surrounding the screwdriver shaft. In some embodiments,the sleeve is at least partially coupled with the temporary encasement.In some further embodiments, the sleeve is at least partiallyretractable away from the screw. In some embodiments, a coupling of thesleeve with the temporary encasement occurs at a transition region, andcomprises a substantially smooth tubular shell at least partiallysurrounding the screwdriver and bone screw.

DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a screwdriver coupling to an encased bone screw inaccordance with at least one embodiment of the invention.

FIG. 1B illustrates an encased bone screw in accordance with at leastone embodiment of the invention.

FIG. 1C illustrates an encased bone screw bone insertion in accordancewith some embodiments of the invention.

FIG. 2A illustrates a bone screw in accordance with another embodimentof the invention.

FIG. 2B illustrates an encased bone screw in accordance with anotherembodiment of the invention.

FIG. 3A illustrates a screwdriver shaft and bone screw assembly inaccordance with at least one embodiment of the invention.

FIG. 3B illustrates a blown-up region A of the assembly of FIG. 3A inaccordance with at least one embodiment of the invention.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

The following discussion is presented to enable a person skilled in theart to make and use embodiments of the invention. Various modificationsto the illustrated embodiments will be readily apparent to those skilledin the art, and the generic principles herein can be applied to otherembodiments and applications without departing from embodiments of theinvention. Thus, embodiments of the invention are not intended to belimited to embodiments shown, but are to be accorded the widest scopeconsistent with the principles and features disclosed herein. Thefollowing detailed description is to be read with reference to thefigures, in which like elements in different figures have like referencenumerals. The figures, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope ofembodiments of the invention. Skilled artisans will recognize theexamples provided herein have many useful alternatives that fall withinthe scope of embodiments of the invention.

Some embodiments of the invention provide a modified bone screw 100 toease percutaneous insertion. For example, some embodiments of theinvention provide a bone screw 100 with a temporarily altered outersurface. FIG. 1A illustrates a screwdriver 150 coupling to an encasedbone screw 100 in accordance with at least one embodiment of theinvention, and FIG. 1B illustrates an encased bone screw 100 inaccordance with a further embodiment of the invention. As shown in FIG.1A, and in more detail in FIG. 1B, in some embodiments, the bone screw100 can be manufactured with a temporary encasement 110 providing anencased bone screw 100. In some embodiments, the temporary encasement110 can provide a temporarily altered outer surface surrounding thescrew shaft 105 that may result in a temporarily modified length todiameter ratio. In some embodiments, the shape, symmetry and/or volumeof the bone screw 100 is temporarily altered using the temporaryencasement 110 covering the bone screw shaft 105 in order to at leastpartially reduce soft tissue interaction during insertion into a patient(e.g., when using a bone screwdriver 150).

For example, in some embodiments, the encasement 110 can be shaped toprovide a spear-shaped volume around the bone screw shaft 105. In someother embodiments, the temporary encasement 110 can be shaped to providevolumes around the bone screw shaft 105 that can be rod-shaped,bullet-shaped, or knife-shaped. In some embodiments, the temporaryencasement 110 can be shaped around the bone screw shaft 105 to form avolume of the bone screw 100 with a substantially symmetrically pointedcurved outer surface.

As depicted in FIG. 1A, in some embodiments, a screwdriver 150 can becoupled with an encased bone screw 100. In some embodiments, thescrewdriver 150 can include a screwdriver shaft 120 including a distalend 120 a and a proximal end 120 b. As shown, the screwdriver 150 caninclude a handle 125 coupled to the proximal end 120 b of the shaft 120.In some embodiments, the distal end 120 a of the screwdriver shaft 120can be coupled with the screw shaft 105 at a screw head 105 a. In someembodiments, a temporary encasement 110 can at least partially envelopthe screw shaft 105. For example, as shown in FIG. 1B, in someembodiments, a temporary encasement 110 can at least partially envelopthe screw 100 covering at least the screw shaft 105 including the screwthread 105 c, the screw head 105 a, and the screw tip 105 b.

In some further embodiments, the temporary encasement 110 may envelopmore or less of the screw 100. For example, in some embodiments, atemporary encasement 110 can at least partially envelop the screw 100covering at least the screw shaft 105 including the screw thread 105 c,and the screw tip 105 b, without enveloping the screw head 105 a. Inother embodiments, a temporary encasement 110 can at least partiallyenvelop the screw 100 covering at least the screw shaft 105 includingthe screw thread 105 c, and the screw head 105 a without enveloping thescrew tip 105 b. In some further embodiments, the temporary encasement110 can at least partially envelop the screw thread 105 c only.

In some embodiments, the thickness of the temporary encasement 110 canbe substantially constant. For example, in some embodiments, thethickness of the temporary encasement 110 can form a substantiallyconstant thickness envelope over the screw 100 covering at least thescrew shaft 105 including the screw thread 105 c, the screw head 105 a,and the screw tip 105 b. In some other embodiments, the thickness of thetemporary encasement 110 may be substantially non-uniform. In someembodiments, the thickness of the temporary encasement 110 may besubstantially constant over at least one region of the screw 100 butdifferent from another region of the screw 100. In some otherembodiments, the thickness of the temporary encasement 110 may besubstantially non-uniform within a region of the screw 100. For example,in some embodiments, the thickness of the temporary encasement 100 maybe substantially non-uniform over the screw shaft 105 including thescrew thread 105 c, the screw head 105 a, and the screw tip 105 b.

In some embodiments, the material properties of the temporary encasement110 (including, but not limited to, the density, hardness, andbrittleness) can be substantially constant. For example, in someembodiments, the material properties of the temporary encasement 110 canbe substantially constant over the screw 100 covering at least the screwshaft 105 including the screw thread 105 c, the screw head 105 a, andthe screw tip 105 b. In some other embodiments, the material propertiesof the temporary encasement 110 may be substantially non-uniform. Insome embodiments, the material properties of the temporary encasement110 may be substantially constant over at least one region of the screw100, but different from another region of the screw 100. In some otherembodiments, the material properties of the temporary encasement 110 maybe substantially non-uniform within a region of the screw 100. Forexample, in some embodiments, the material properties of the temporaryencasement 110 may be substantially non-uniform over the screw shaft 105including the screw thread 105 c, the screw head 105 a, and the screwtip 105 b. In some further embodiments, the material properties of thetemporary encasement 110 can be most dense, hard, and brittle at thescrew tip 105 b and less dense, hard, and brittle in regions of thetemporary encasement enveloping other portions of the screw 100.

In some embodiments, the temporary encasement 110 may comprise a coatingthat forms a smooth, substantially bullet-shaped surface over at leastthe screw threads 105 c. In some embodiments, the temporary encasement110 can include a substantially smooth outer surface 110 a. In someembodiments, the temporary encasement 110 may have sufficient hardnessto allow the bone screw 100 to penetrate through soft tissues such asmuscle substantially intact, but soft and/or brittle enough to allow atleast a portion of the temporary encasement 110 to crumble and/orsubstantially depart from one or more of the screw shaft 105, the screwthread 105 c and the screw tip 105 b, and the screw head 105 a duringpenetration into bone. For example, FIG. 1C illustrates an encased bonescrew 100 bone insertion in accordance with some embodiments of theinvention. In some embodiments, when the screwdriver 150 is coupled withthe bone screw 100 (as depicted in FIG. 1A) and driven into a patient,the temporary encasement 110 may crumble and/or extrude away as thescrew penetrates bone leaving encasement debris 140. In someembodiments, substantially all the temporary encasement 110 may bestripped from any region of screw 100 that enters the bone.

In some other embodiments, the temporary encasement 110 may be at leastpartially removed from the screw 100 upon entering the bone (i.e., atleast some fraction of the temporary encasement 110 may remain at aninterface between the bone and the screw 100). In some embodiments, whenthe temporary encasement 110 comprises a material that comprises anosteoinductive, and/or hemostatic, and/or bacteriocidal, after drivingthe screw into bone, the presence of residual material (debris 140),derived from the encasement 110 in and around the screw-bone interface,may provide therapeutic value. Moreover, in some embodiments, at leastsome of the temporary encasement 110 may remain on the screw 100, andmay provide some lubricating effect.

Some embodiments can include bone screws with alternative geometries.For example, FIG. 2A illustrates a bone screw 200 in accordance withanother embodiment of the invention, and FIG. 2B illustrates the bonescrew 205 (the bone screw 200 including temporary encasement 217). Asshown, screws 200, 205 can be especially suited to percutaneousimplantation. For example, in some embodiments, screws 200, 205 caninclude a leading small-diameter tip 220 b that can penetrate bone andmore easily hold a desired trajectory on a contoured complex surface(i.e., without wandering across the bone surface) than a standard screwtip (e.g., a screw 200 without the extension tip 220). The screws 200,205 can then be advanced and inserted into bone, either with or withouta previously drilled pilot hole. As shown, in some embodiments, the bonescrews 200, 205 comprising a distal end 202 a and a proximal end 202 b,and the bone screw 205 comprising a distal end 207 a and a proximal end207 b, can include a screw shaft 210 comprising a distal end 210 a and aproximal end 210 b coupled to a U-shaped screw head 212 including aninner surface 212 a. In some embodiments, screws 200, 205 also comprisea screw thread 215 coupled to an extension tip 220 with tip end 220 b,and a shaft 220 d and a proximal end 220 a. As shown, in someembodiments, the U-shaped screw head 212 is wider than the shaft 220 d,as is typical for standard top-loading pedicle screws into whichinterconnecting rods are locked after the screw has been inserted.

In some embodiments, the tip end 220 b can include a tip taper 220 c,and the shaft 220 d can comprise a screw thread 220 e. In someembodiments, the screws 200, 205 can comprise a standard diameterbetween about 3.5 mm and 7 mm. In some embodiments, the shaft 220 d canbe between about 1 mm to about 2 mm in diameter and the tip end 220 bcan extend between about 2 mm and about 4 mm from the proximal end 220a. In some other embodiments, the screw 200, 205 may have a diametergreater than about 7 mm, whereas in other embodiments, the screw 200,205 may have a diameter less than about 3.5 mm. In some embodiments, theshaft 220 d may have a diameter less than about 1 mm, whereas in otherembodiments, the diameter may be greater than about 2 mm. Someembodiments include a tip end 220 b that can extend up to about 2 mm,whereas in other embodiments, the tip end 220 b can extend further thanabout 4 mm from the proximal end 220 a.

In some embodiments, the leading small-diameter tip 220 b can penetratebone and hold the desired trajectory on a contoured complex surface moreeasily (i.e., without wandering across the bone surface) than a standardscrew tip (e.g., a screw 200, 205 without the extension tip 220). Thescrew 200, 205 can then be advanced and inserted into bone, either withor without a previously drilled pilot hole.

As shown in FIG. 2B, the bone screw 205 can include a temporaryencasement 217 to aid in streamlining insertion. The wide screw headarchitecture illustrated by the screw head 212 can be difficult toinsert percutaneously because the screw head 212 can become entangledwith body matter while being driven The temporary encasement 217 canenvelop the screw 205 covering at least the shaft 210, and canfacilitate insertion, making it unnecessary to first insert a tubularretractor. In some embodiments as shown, the temporary encasement 217can gradually taper toward the tip 220, ending at the distal end 210 aof the shaft 210, and flaring to a larger diameter toward the head 212.In some further embodiments, the temporary encasement 217 may envelopmore or less of the screw 205. For example, in some embodiments, atemporary encasement 217 can at least partially envelop the screw 205covering at least the screw shaft 210 including the screw thread 215,and the distal end 210 a. In other embodiments, the temporary encasement217 can at least partially envelop the screw 200 covering at least thescrew shaft 210 including the screw thread 215, and the screw head 212without enveloping the inner surface 212 a. In some further embodiments,the temporary encasement 217 can at least partially envelop the screwthread 215 only. In some other embodiments, the temporary encasement 217can at least partially envelop the extension tip 220.

In some embodiments, the thickness of the temporary encasement 217 canbe substantially constant. For example, in some embodiments, thethickness of the temporary encasement 217 can be a substantiallyconstant thickness envelope over the screw 205, covering at least thescrew shaft 210 including the screw thread 215, the screw head 212, andthe distal end 210 a. In some other embodiments, the thickness of thetemporary encasement 217 may be substantially non-uniform. In someembodiments, the thickness of the temporary encasement 217 may besubstantially constant over at least one region of the screw 205, butdifferent from another region of the screw 205. In some otherembodiments, the thickness of the temporary encasement 217 may besubstantially non-uniform within a region of the screw 205. For example,in some embodiments, the thickness of the temporary encasement 217 maybe substantially non-uniform over the screw shaft 210, including thescrew thread 215, the screw head 212, and the distal end 210 a.

In some embodiments, the material properties, including but not limitedto density, brittleness, and hardness, of the temporary encasement 217can be substantially constant. For example, in some embodiments, thedensity of the temporary encasement 217 can be substantially constantover the screw 205, covering at least the screw shaft 210 including thescrew thread 215, the screw head 212, and the distal end 210 a. In someother embodiments, the density of the temporary encasement 217 may besubstantially non-uniform. In some embodiments, the density of thetemporary encasement 217 may be substantially constant over at least oneregion of the screw 205, but different from another region of the screw205. In some other embodiments, the density of the temporary encasement217 may be substantially non-uniform within a region of the screw 205.For example, in some embodiments, the density of the temporaryencasement 217 may be substantially non-uniform over the screw shaft 210including the screw thread 215, the screw head 212, and the distal end210 a. In some embodiments, the density of the temporary encasement 217is most dense at the screw tip and less dense in other portions of thescrew 205.

In some embodiments, the temporary encasement 110, 217 can comprise awax. In some embodiments, the temporary encasement 110, 217 may be softand/or brittle enough that the temporary encasement 110, 217 crumblesaway as the screw penetrates bone. In some embodiments, the temporaryencasement 110, 217 can comprise a material that is biocompatible,osteoinductive, and/or hemostatic, and/or bacteriocidal. Further, insome embodiments, a therapeutic compound may be dispersed within thetemporary encasement 110, 217, or the temporary encasement 110, 217 maycomprise a therapeutic compound.

In some embodiments, the wax can comprise a hydrocarbon-based wax. Someembodiments include wax comprising an ester of a monohydric long-chainfatty alcohol and a long chain fatty acid. In some embodiments, thetemporary encasement 110, 217 can comprise natural waxes, including, butnot limited to animal-derived waxes such as lanoline, spermaceti wax,and wool fat, insect waxes such as bees wax, and vegetable-derived waxsuch as candelila wax, carnauba wax or castor wax. In some otherembodiments, the temporary encasement 110, 217 can comprise a syntheticwax. For example, some embodiments can include poly-ethyleneglycol-based wax, hydrogenated or partially hydrogenated vegetable oilbased wax. Some embodiments of the invention include a temporaryencasement 110, 217 comprising at least one biocompatible wax. Forexample, some embodiments can include at least biodegradable orbio-absorbable wax. Some embodiments include a temporary encasement 110,217 comprising a non-waxy biocompatible semi-crystalline or amorphouspolymer.

In some embodiments, the temporary encasement 110, 217 includes acoating with a melting temperature of about 98° F. (i.e., bodytemperature). In some other embodiments, the temporary encasement 110,217 includes a coating with a melting temperature greater than 98° F.,whereas in other embodiments, the temporary encasement 110, 217 includesa coating with a melting temperature of less than 98° F. In someembodiments, the temporary encasement 110, 217 comprises a material witha melting or softening point that is selected based on the length oftime of the procedure. For example, in some embodiments, the temporaryencasement 110, 217 comprises a material that is substantially solid atroom temperature prior to entering patient, but later becomes softerand/or enters a melting phase as its temperature is raised by the bodyof the patient during the procedure. In some embodiments, the temporaryencasement 110, 217 comprises a material that is substantially solid atroom temperature prior to entering the patient and during percutaneousinsertion, but becomes softer and/or enters a melting phase as itstemperature is raised as the bone screw 100 enters the bone of thepatient.

In some embodiments of the invention, the bone screw 100, 200, 205 cancomprise a biocompatible metal. In some embodiments, the biocompatiblemetal can be stainless steel, such as a surgical stainless steel. Inother embodiments, other metals or metal alloys can be used based on atleast one of iron, chromium, nickel, molybdenum, titanium or other groupIV metal. In other embodiments, the bone screw 100, 200, 205 cancomprise a polymer, a ceramic, a glass, a metal-matrix composite, orcombinations thereof.

FIG. 3A illustrates a screwdriver 300 and bone screw 200 assembly inaccordance with at least one embodiment of the invention, and FIG. 3Billustrates a blown-up region A of the assembly of FIG. 3A in accordancewith at least one embodiment of the invention. FIG. 3B shows shaft 312coupled with the inner surface 212 a of the screw head 212. In someembodiments, the shaft 312 can pass through a plastic or metal sleeve310. In some embodiments, the plastic or metal sleeve 310 can couplewith the temporary encasement 217 at a transition region 315 to form asmooth tubular shell surrounding the screwdriver 300 and bone screw 205.In some embodiments, the plastic or metal sleeve 310 is at leastpartially retractable away from the bone screw 200 and the transitionregion 315.

In some embodiments, any one of the bone screws 100, 105, 200, 205described above and illustrated in FIGS. 1A-1C, 2A-2B, and 3A-3B may bedriven by a medically qualified individual such as a surgeon, physician,or a physician assistant. In some further embodiments, any one of thebone screws 100, 105, 200, 205 described above and illustrated in FIGS.1A-1C, 2A-2B, and 3A-3B may be driven by a surgical robot (andoptionally an imaging system) that utilizes a Cartesian positioningsystem that allows movement of a surgical instrument to be individuallycontrolled in an x-axis, y-axis and z-axis. A surgical robot suitablefor this task has been described in U.S. Provisional Patent ApplicationNo. 61/662,702 filed on Jun. 21, 2012, U.S. Provisional PatentApplication No. 61/800,527 filed on Mar. 15, 2013, and Non-Provisionalpatent application Ser. No. 13/924,505 filed on Jun. 21, 2013, theentire contents of which are hereby incorporated by reference. Asdescribed, the surgical robot can include a base, a robot arm coupled toand configured for articulation relative to the base, as well as anend-effectuator coupled to a distal end of the robot arm. Theeffectuator element can include the surgical instrument or can beconfigured for operative coupling to the surgical instrument, and theroll, pitch and yaw rotation of the end-effectuator and/or surgicalinstrument to be controlled without creating movement along the x-axis,y-axis, or z-axis. The system can be configured to automaticallyposition and rigidly hold the end-effectuator and/or the surgicalinstrument in accurate alignment with a required trajectory, such as,for example, a selected trajectory of a pedicle screw during pediclescrew insertion procedures. In case of movement of the patient, thesystem can be configured to automatically adjust the position of therobot to maintain desired alignment relative to an anatomical region ofinterest.

In some embodiments, when the screwdriver 300 is coupled with the bonescrew 205 (as depicted in FIG. 3A) and driven into a patient, thetemporary encasement 217 may crumble and/or extrude away as the screw205 penetrates bone leaving encasement debris (illustrated by 140 inFIG. 1C). In some embodiments, substantially all the temporaryencasement 217 may be stripped from any region of screw 205 that entersthe bone. In some other embodiments, the temporary encasement 217 may beat least partially removed from the screw 205 upon entering the bone. Insome embodiments, when the temporary encasement 217 comprises a materialthat comprises an osteoinductive, and/or hemostatic, and/orbacteriocidal, after driving the screw 205 into bone, the presence ofresidual material (debris 140), derived from the encasement 217 in andaround the screw-bone interface, may provide therapeutic value. In someembodiments, some of the temporary encasement 217 remains on the screw205 and provides some lubricating effect.

In some embodiments, any one of the bone screws 100, 200, 205 describedabove and illustrated in FIGS. 1A-1C, 2A-2B, and 3A-3B can be used fortherapy within any one of the cervical region of the human spine, thethoracic region of the human spine, the lumbar region of the humanspine, and the sacral region of the human spine.

As described earlier, in some embodiments, when any one of the bonescrews 100, 200, 205 described above and illustrated in FIGS. 1A-1C,2A-2B, and 3A-3B is turned and driven into bone, the temporaryencasement 110, 217 can be at least partially shed (i.e., the materialmay at least partially crumble, if the material is brittle, or thematerial may at least partially flow if material is softening orentering a melting phase). In some other embodiments, the temporaryencasement 110, 217 can be removed by thermal-induced melting and/orsoftening. For example, in some embodiments a conventional heatingelement can be included in the screwdriver 150, 300 just proximal to thescrew 100, 205 to heat the screw 100, 205 to induce melting and/orsoftening of the temporary encasement 110, 217 on the screw 100, 205. Insome other embodiments, an electrical current can be used to heat thescrew 100, 205 to induce melting and/or softening of the temporaryencasement 110, 217 on the screw 100, 205.

Some other embodiments can include chemically-induced removal of thetemporary encasement 110, 217. For example, some embodiments can includeinjection of a chemical through a cannulation in a shaft of thescrewdriver 150, 300, and/or fenestrations or porosity in the screw 100,200, 205. In some embodiments, the chemical may at least partiallydissolve and/or penetrate at least some fraction of the temporaryencasement 110, 217. In some other embodiments, the chemical may atleast partially react with at least some fraction of the temporaryencasement 110, 217. In some embodiments, the chemical may soften thetemporary encasement 110, 217. In some embodiments, contact between anyfraction of the temporary encasement 110, 217 may cause the temporaryencasement 110, 217 to at least partially separate from the screw 100,200, 205.

The invention claimed is:
 1. A bone screw device comprising: a screwcomprising a screw shaft, a screw head and screw tip coupled to thescrew shaft, the screw shaft including a screw thread; and a temporaryencasement at least partially covering the screw, wherein the temporaryencasement crumbles or extrudes away from the screw during penetrationinto bone, thereby leaving encasement debris and the screw with at leasta portion of the temporary encasement removed.
 2. The bone screw deviceof claim 1, wherein the temporary encasement envelops substantially allof the screw thread.
 3. The bone screw device of claim 2, wherein thetemporary encasement comprises a substantially symmetrically curvedsurface extending over the screw tip.
 4. The bone screw device of claim1, wherein the temporary encasement comprises a biocompatible material.5. The bone screw device of claim 4, wherein the biocompatible materialis at least one of an osteoinductive, a hemostatic and a bacteriocide.6. The bone screw device of claim 1, wherein the temporary encasementcomprises a therapeutic compound.
 7. The bone screw device of claim 6,wherein the therapeutic compound is dispersed within the temporaryencasement.
 8. The bone screw device of claim 1, wherein the temporaryencasement comprises a wax.
 9. The bone screw device of claim 1, whereinthe screw shaft further includes: a shaft proximal end and a shaftdistal end; and wherein the screw head is substantially U-shaped andcoupled to the shaft proximal end.
 10. The bone screw device of claim 9,further comprising: an extension tip comprising a tip shaft including ascrew thread, and a tip proximal end coupled to the bone screw distalend, and a tip end extending from the shaft distal from the bone screwdistal end.
 11. A system for percutaneous implantation of a bone screw,comprising: a screwdriver having a sleeve and a shaft passing throughthe sleeve; a bone screw comprising a proximal end and a distal end andincluding a screw shaft, a screw head, and a screw tip coupled to thescrew shaft, the screw shaft comprising a proximal shaft end and adistal shaft end and including a screw thread; and a temporaryencasement at least partially covering the screw; wherein the screwshaft is configured and arranged to be at least partially rotated withinthe sleeve by the screwdriver, and wherein the temporary encasementcrumbles or extrudes away from the bone screw during penetration intobone, thereby leaving encasement debris and the bone screw with at leasta portion of the temporary encasement removed.
 12. The system of claim11, wherein the screw head is substantially U-shaped and coupled to theproximal screw shaft end; and wherein the screwdriver is coupled to aninner surface of the screw head.
 13. The system of claim 11, wherein thesleeve is a substantially rigid sleeve at least partially surroundingthe screwdriver shaft.
 14. The system of claim 13, wherein the sleeve isat least partially coupled with the temporary encasement.
 15. The systemof claim 11, wherein the sleeve is at least partially retractable awayfrom the screw.
 16. The system of claim 14, wherein the coupling of thesleeve with the temporary encasement occurs at a transition region andcomprises a substantially smooth tubular shell at least partiallysurrounding the screwdriver and bone screw.
 17. The bone screw device ofclaim 11, wherein the temporary encasement comprises a biocompatiblematerial.
 18. The bone screw device of claim 17, wherein thebiocompatible material is at least one of an osteoinductive, ahemostatic and a bacteriocide.
 19. The bone screw device of claim 17,wherein the biocompatible material comprises a therapeutic compound. 20.The bone screw device of claim 17, wherein the biocompatible materialcomprises a wax.